Omniphobic Surface Coatings

Abstract

The disclosure relates to omniphobic surface coatings including a solution of fluor-modified polymer and crystalline and/or semi-crystalline polymer and/or inorganic nanoparticles. The disclosure further relates to biphilic substrate surfaces for heat exchangers, including 50-95% of the surface showing a first solid-liquid contact angle and 5 to 50% of the surface showing a second solid-liquid contact angle, wherein the second liquid-solid contact angle is at least 10 higher than first liquid-solid contact angle, and the surface area of second contact angle includes a multitude of discrete surface areas of second contact angle dispersed over the substrate surface.

Claims

1. An omniphobic surface coating comprising a polymer, fluorine molecules or radicals dispersed therein, and microparticles or nanoparticles of crystallized crystalline and/or semi-crystalline polymer dispersed therein and/or other nanoparticles dispersed therein.

2. The omniphobic surface coating of claim 1, comprising a fluor-modified polymer and microparticles or nanoparticles of crystallized crystalline and/or semi-crystalline polymer dispersed therein and/or other nanoparticles dispersed therein.

3-34. (canceled)

35. The omniphobic surface coating of claim 1, wherein the fluor-modified polymers are based on fluorinated epoxy based polymers, preferably high and low molecular weight epoxy resins curable by homopolymerisation or with a curing agent (or hardener) selected from polyfunctional amines, acids, alcohols and thiols, preferably phenol based epoxy polymers, most preferably selected from bisphenol A epoxy resin, bisphenol F epoxy resin, novolac epoxy resin, for example a biobased epoxydized material obtained from cardanol, such as NC-514 cardanol based epoxy polymers, perfluoroalkene, perfluorocycloalkene, fluoroethylene, vinylfluoride, vinylidene fluoride, tetrafluoroethylene, chlorotrifluoroethylene, fluoropropylene, perfluoropropylvinylether, perfluoromethylvinylether or copolymers thereof.

36. The omniphobic surface coating of claim 1, wherein the crystalline and/or semi-crystalline polymer and/or the nanoparticles are present in a weight ratio to the fluor-modified polymer such that the polymer composition shows enhanced omniphobic properties.

37. The omniphobic surface coating of claim 36, wherein the ratio ranges from 20:80 to 80:20, preferably from 25:75 to 75:25, or 25:70 to 50:50.

38. The omniphobic surface coating of claim 1, wherein the crystalline and/or semi-crystalline polymer is selected from polypropylene (PP), preferably isotactic polypropylene, carnauba wax, polycarbonate (PC), polymethylmethacrylate (PMMA), polylactic acid (PLA), polyhydroxyalkanoates (PHA), polyhydroxybutyrate (PHB), polyamide (PA 11, PA 410), starch-based plastics, cellulose-based plastics, and fibrin-based plastics.

39. The omniphobic surface coating of claim 1, wherein the crystalline and/or semi-crystalline polymer includes homopolymers, copolymers, such as ethylene-propylene block copolymers, random copolymers, graft copolymers, such as polypropylene or polylactic acid grafted with maleic anhydride or acrylic acid, halogenated polymers, surface oxidized polymers, and other modifications known to the skilled person.

40. The omniphobic surface coating of claim 1 wherein the molecular weight of the crystalline or semi-crystalline polymer varies within a range of molecular weights of 1000 to 1000000 Da, preferably between 2000 and 200000 or more preferably between 2500 and 100000 Da.

41. An omniphobic material comprising an epoxy-based polymer and fluorine molecules or radicals dispersed therein, wherein the epoxy-based polymer is selected from bio-based epoxydized material obtained from cardanol curable with a curing agent (or hardener) selected from polyfunctional amines, acids, alcohols and thiols, preferably NC-514.

42. The omniphobic material of claim 41, wherein fluorine is grafted onto the epoxy-based polymer.

43. An omniphobic coating composition comprising a solution of fluor-modified polymer and crystalline and/or semi-crystalline polymer and/or other nanoparticles.

44. The omniphobic coating composition of claim 43, wherein the solvent is selected from xylene, a xylene based solvent system, methyl ethyl ketone, tetrahydrofuran, toluene, dibasic esters, DMSO, limonene, butylal or a mixture thereof.

45. The omniphobic coating composition of claim 43, comprising the crystalline and/or semi-crystalline polymer in a weight ratio to the fluor-modified polymer of 20:80 to 80:20, preferably 25:75 to 75:25 or 25:70 to 50:50.

46. The omniphobic coating composition of claim 43, wherein the crystalline and/or semi-crystalline polymer is selected from polypropylene (PP), preferably isotactic polypropylene, carnauba wax, polycarbonate (PC), polymethylmethacrylate (PMMA), polylactic acid (PLA), polyhydroxyalkanoates (PHA), polyhydroxybutyrate (PHB), polyamide (PA 11, PA 410), starch-based plastics, cellulose-based plastics, and fibrin-based plastics.

47. The omniphobic coating composition of claim 43, wherein the fluor-modified polymers are based on perfluoroalkene, perfluorocycloalkene, fluoroethylene, vinylfluoride, vinylidene fluoride, tetrafluoroethylene, chlorotrifluoroethylene, fluoropropylene, perfluoropropylvinylether, perfluoromethylvinylether or copolymers thereof.

48. A process for the preparation of an omniphobic surface coating, comprising applying an omniphobic coating composition according to claim 43 onto a surface, and allowing for solvent evaporation under suitable conditions for crystal rearrangement.

49. The process of claim 48, wherein the solvent evaporation is effected at a minimum temperature of about 15 C. below the melting point of the crystalline and/or semi-crystalline polymer, preferably at a minimum temperature of about 10 C. below the melting point of the crystalline and/or semi-crystalline polymer, more preferably a minimum temperature of about 5 C. below the melting point of the crystalline and/or semi-crystalline polymer., and at a maximum temperature such as to allow for rearrangement of the crystal structure of the crystalline and/or semi-crystalline polymer and formation of nanoparticles and/or microparticles of crystallized crystalline or semi-crystalline polymer of 25 C. beyond the melting point of the crystalline or semi-crystalline polymer, preferable 15 C. beyond the melting point of the crystalline or semi-crystalline polymer, in a temperature range of from 5-10 C. below to 5-10 C. above melting point of the relevant crystalline or semi-crystalline polymer in the solution.

50. The process of claim 48, wherein the process steps are repeated, preferably up to two to three times.

51. The process of claim 48, wherein the coating obtained is overcoated with a layer of epoxy resin, preferably an epoxy resin derived from cardanol, such as NC-514, possibly fluorinated.

52. A biphilic substrate surface, such as for instance a heat exchanging surface of a pool boiling heat exchanger, comprising 50.0-99.9% of the surface showing a first degree of wettability defined by a first liquid-solid contact angle and 0.1 to 50.0% of the surface showing a second degree of wettability to the said liquid, wherein the second degree of wettability is defined by a second liquid-solid contact angle at least 10 higher than first liquid-solid contact angle, and the surface area of second degree of wettability comprising a multitude of discrete surface areas of second degree of wettability dispersed over the substrate surface, wherein the surface area showing the second degree of wettability is formed by a surface material selected from (i) a polymer material comprising a matrix of amorphous polymer showing a contact angle with said liquid higher than 15, preferably higher than 25 or higher than 35 or 45, more preferably higher than 55 or 65, even more preferably higher than 75 or 85, more particularly higher than 90 and microparticles or nanoparticles of crystallized crystalline and/or semi-crystalline polymer dispersed therein, wherein the crystalline and/or semi-crystalline polymer is present in a weight ratio to said amorphous matrix polymer such that the polymer surface material shows a significantly increased value for the contact angle to said liquid, and wherein the crystalline and/or semi-crystalline polymer is selected from polypropylene (PP), preferably isotactic polypropylene, carnauba wax, polycarbonate (PC), polymethylmethacrylate (PMMA), polylactic acid (PLA), polyhydroxyalkanoates (PHA), polyhydroxybutyrate (PHB), polyamide (PA 11, PA 410), starch-based plastics, cellulose-based plastics, and fibrin-based plastics; (ii) a polymer material comprising a matrix of amorphous polymer showing a contact angle higher than 15, preferably higher than 25 or higher than 35 or 45, more preferably higher than 55 or 65, even more preferably higher than 75 or 85, more particularly higher than 90 and nanoparticles; or (iii) fluorine-modified epoxy-based polymer.

53. The biphilic substrate surface of claim 52, wherein the surface material comprises the crystalline and/or semi-crystalline polymer in a weight ratio to the amorphous matrix polymer of 20:80 to 80:20, preferably 25:75 to 75:25, or 25:70 to 50:50, and in such proportion that the polymer composition shows significantly increased contact angle.

54. The biphilic substrate surface of claim 52, wherein the surface material comprises nanoparticles in a weight ratio to amorphous polymer of 20:80 to 80:20, preferably 25:75 to 75:25, or 25:70 to 50:50, and in such proportion that the polymer composition shows significantly increased contact angle.

55. The biphilic substrate surface of claim 52, wherein the amorphous matrix polymer is selected from polystyrene (PS), polyethylene (PE), preferably low density polyethylene (LDPE), and polychloroprene (PCP), and from polymers which do not show a high interface contact angle (higher than 15, 25, 35, 45, 55, 65, 75, 85 or 90 with relevant liquid by themselves but which are functionalized such as to show high contact angle, like polyurethane (PU), polyvinylacetate (PVA), polyacrylic acid, polyacrylate, and epoxy resins.

56. The biphilic substrate surface of claim 52, wherein the crystalline and/or semi-crystalline polymer is polypropylene, preferably isotactic polypropylene.

57. The biphilic substrate surface of claim 52, wherein the crystalline and/or semi-crystalline polymer comprises homopolymers, copolymers, such as ethylene-propylene block copolymers, random copolymers, graft copolymers, such as polypropylene or polylactic acid grafted with maleic anhydride or acrylic acid, halogenated polymers, surface oxidized polymers, and other modifications known to the skilled person.

58. The biphilic substrate surface of claim 52, wherein the weight of the crystalline or semi-crystalline polymer varies within a range of molecular weights of from 1000 to 1000000 g/mol, preferably between 5000 and 500000 or more preferably between 5000 and 300000 g/mol.

59. The biphilic substrate surface of claim 52, wherein the nanoparticles are organic or inorganic or a mixture thereof, possibly treated or functionalized for increased interface contact angle with said liquid, advantageously inorganic nanoparticles, preferably selected from metal oxides, SiO2 or TiO2.

60. The biphilic substrate surface of claim 52, wherein the amorphous matrix polymer comprises an epoxy resin showing an interface contact angle of more than 35, preferably more than 55, more than 65 or more than 75 or 85 or even 90 with the said liquid.

61. The biphilic substrate surface of claim 52, wherein the amorphous hydrophobic matrix polymer comprises an epoxy resin rendered hydrophobic by chemical modification, crosslinking or other methods known per se, such as bisphenol A epoxy resin, bisphenol F epoxy resin, novolac epoxy resin, a biobased epoxydized material obtained from cardanol, for example NC-514.

62. The biphilic substrate surface of claim 52, wherein the difference of contact angle is at least 20, more preferably at least 30, more preferably at least 40, more preferably at least 50, more preferably at least 60, more preferably at least 70, for example at least 80, at least 90, at least 100, at least 120, at least 150.

63. The biphilic substrate surface of claim 52, wherein the surface material of second degree of wettability is a coating applied onto and bonded to the substrate surface by way of an intermediate binding layer.

64. The biphilic substrate surface of claim 52, wherein the surface area showing the first degree of wettability is an untreated or treated metallic surface, preferably with a surface roughness below 1 m, such as for instance stainless steel or aluminium or copper, or a substrate surface coated with a coating that shows the required wettability character.

65. A process for the manufacture of a biphilic substrate surface according to claim 54, comprising spraying a solution of the surface material polymer of second degree of wettability as discrete areas over a substrate surface at a distance from the target surface and at a rate such as to spray spots of said surface material polymer of second wettability degree onto the substrate target surface, the total surface of second degree of wettability being 5 to 50% of the total substrate surface.

66. The process of claim 65, wherein the solvent is selected from xylene, a xylene based solvent system, methyl ethyl ketone, DMSO, limonene, butylal or a mixture thereof.

Description

[0068] The present invention will be described in more details below, by way of example only, with reference to the drawings of which

[0069] FIG. 1 shows image processing to evaluate grain size and distribution on the surface; and

[0070] FIG. 2 is a schematic representation of the test set up and procedure;

[0071] FIG. 3 is a graphic representation of average grains size, average minimum distance and percentage superhydrophobic surface coverage;

[0072] FIG. 4 shows T.sub.ONB for the various coatings evaluated in Example 3;

[0073] FIG. 5 shows the heat flux as a function of wall temperature during first and second cycle for tested surface coatings;

[0074] FIG. 6 graphically represents T.sub.ONB for the coatings of Example 4 in water;

[0075] FIG. 7 is a schematic representation of a methanol pool boiling test apparatus;

[0076] FIG. 8 graphically represents T.sub.ONB for coatings in methanol;

[0077] FIG. 9 shows experimental set up and temperature cycle in thermal cycling conditions; and

[0078] FIG. 10 shows T.sub.ONB and Equilibrium Static Contact Angle for coatings of Example 6

EXAMPLE 1

[0079] The following solutions were prepared:

[0080] Superhydrophobic polymeric composition (OPSPP/epoxy suspension or dispersion):

[0081] In order to prepare an epoxy solution containing 30 wt % of Polypropylene (PP), 1.7 g of PP and 3.61 g of NC514 were dissolved in 50 ml xylene and heated under reflux at 135 C. under continuous stirring until a homogeneous solution was obtained. Next, an amine monomer (IPDA) dissolved in 5 ml xylene was combined with the PP solution at room temperature and mixed at 12000 rpm in a high velocity homogenizer (SilentCrusher M from Heidolph) for 5 min.

[0082] Fluorinated superhydrophobic polymer solution (about 5 wt % F in host polymerF5OPS):

[0083] First, a partially fluorinated amine monomer was prepared by reacting 0.125 ml of fluorinated epoxy oligomer (heptadecafluorononyl oxirane, Sigma-Aldrich) with a known excess of 0.618 ml of IPDA at about 100 C. for 120 min, in a sealed tube.

[0084] Next, in order to prepare a fluorinated epoxy solution containing 30 wt % of Polypropylene (PP), 2.2 g of PP and 4.305 g of NC514 were dissolved in 64 ml xylene and heated under reflux at 135 C. under continuous stirring until a homogeneous solution was obtained. Thereafter, the previous solution of partially fluorinated amine monomer dissolved in 8 ml THF was combined with the PP solution at room temperature and mixed at 12000 rpm in a high velocity homogenizer (SilentCrusher M from Heidolph) for 5 min.

[0085] Neat cardanol (SC):

[0086] A solution of epoxy cardanol was prepared by mixing 3.61 g NC514 and 0.46 g IPDA with 15 ml xylene at RT until a homogeneous solution was obtained (using manual mixing by spatula).

[0087] Preparation of fluorinated epoxy (about 15 wt % F in host polymer)FC15:

[0088] First, a partially fluorinated amine monomer was prepared by reaction of 0.343 ml of fluorinated epoxy oligomer (heptadecafluorononyl oxirane, Sigma-Aldrich) with a known excess of 0.609 ml of IPDA at about 100 C. for 120 min, in a sealed tube. Thereafter, the previous solution of partially fluorinated amine monomer was dissolved in 5 ml toluene and 5 ml THF. 3.936 g NC514 and 20 ml xylene were mixed at room temperature until a homogeneous solution was obtained (using manual mixing by spatula). The solution of partially fluorinated amine monomer was added to the previous solution and mixed (using manual mixing by spatula).

[0089] Preparation of fluorinated epoxy (about 5 wt % F in host polymer) FC5:

[0090] First, a partially fluorinated amine monomer was prepared by reaction of 0.250 ml of fluorinated epoxy oligomer (heptadecafluorononyl oxirane, Sigma-Aldrich) with a known excess of 1.236 ml of IPDA at about 100 C. for 120 min, in a sealed tube.

[0091] Thereafter, the previous solution of partially fluorinated amine monomer was dissolved in 8 ml toluene and 2 ml THF. Next, 8.62 g NC514 and 200 ml xylene were mixed at room temperature until a homogeneous solution was obtained (using manual mixing by spatula). Further, the solution of partially fluorinated amine monomer was added to the previous solution and mixed (using manual mixing by spatula).

[0092] Preparation of fluorinated epoxy (about 5 wt % F in host polymer) containing 37 wt % nanoparticles (FC5NP37):

[0093] A partially fluorinated amine monomer was prepared by reaction of 0.250 ml of fluorinated epoxy oligomer (heptadecafluorononyl oxirane, Sigma-Aldrich) with a known excess of 1.236 ml of IPDA at about 100 C. for 120 min, in a sealed tube. Thereafter, the previous solution of partially fluorinated amine monomer was dissolved in 8 ml toluene and 2 ml THF. Next, 6.0 g of hydrophobic SiO.sub.2 (HDK18) nanoparticles were manually mixed with 8.62 g NC514 and 200 ml xylene at room temperature and then mixed at 12000 rpm in a high velocity homogenizer (SilentCrusher M from Heidolph) for 1 min. The solution of partially fluorinated amine monomer was added to the previous dispersion of nanoparticles and epoxy and mixed at 12000 rpm in a high velocity homogenizer SilentCrusher M from Heidolph) for 2 min. Finally, the solution was sonicated for 30 min and ultrasonicated at 40% amplitude during 1 min 30 sec.

[0094] Preparation of a multi-layered coating over a steel sample by spraying:

[0095] Several layers of the different solutions were sprayed (Badger Air-Brush 360-Universal) onto the target surface as follows (Sample code: C_OPS_NP_F)

TABLE-US-00001 Spray gun Layer Solution Aliquot (ml) distance 1 FC15 1 30 2 FC5NP37 1 30 3 F5OPS 1 40 4 FC5NP37 1 40 5 FC5 1 40 6 F5OPS 1 40 7 FC5NP37 1 40 8 FC5 1 40 9 F5OPS 1 40 10 FC5NP37 1 40 11 FC5 1 40 12 FC5NP37 1 40 13 FC5 4 40

[0096] The final coating was cured at 80 C. during 24 h. The static contact angle of hexadecane on the final coating was 121.8+/0.7.

[0097] The same experiment was repeated with the following layers (Sample code: C_OPS):

TABLE-US-00002 Spray gun Layer Solution Aliquot (ml) distance 1 SC 1 20 2 OPS 1 30 3 SC 1 40 4 OPS 1 40 5 SC 1 40 6 OPS 1 40 7 SC 1 40

[0098] The final coating was cured at 60 C. during 16 h. The final coating was completely wetted with hexadecaneno contact angle

[0099] The same experiment was repeated with the following layers (Sample code: C_OPS_F):

TABLE-US-00003 Spray gun Layer Solution Aliquot (ml) distance 1 SC 1 40 2 SC 1 40 3 FC5 1 40 4 FC5 1 40 5 F5OPS 1 40 6 FC5 1 30 7 F5OPS 1 40 8 FC5 1 30 9 F5OPS 1 40 10 FC5 4 30

[0100] The final coating was cured at 80 C. for 24 h. The static contact angle of hexadecane on the final coating was 123.7+/1.7.

EXAMPLE 2

[0101] A two neck round bottom flask of 100 ml was charged with 1.7 g of isotactic polypropylene and 40 ml of xylene. The amount of solvent used was varied as shown in Table 1 in order to generate different sizes of crystalline PP grains. The flask was connected to Liebig condenser and a magnetic stirrer introduced into the flask. The flask was heated at 135 C. in an oil bath and the temperature was controlled by a probe sensor in direct contact with the solution. The mixture was heated under reflux under continuous stirring until a homogenous solution was obtained. Thereafter, the solution was cooled at room temperature under stirring.

TABLE-US-00004 TABLE 1 Solutions of polymeric surface material Solution Composition BIG 30 wt % PP by Total, Mw 235000 g/mol in 40 ml Xylene SB 30 wt % PP by Total, Mw 235000 g/mol in 25 ml Xylene

[0102] 3.61 g of NC-514(epoxy-cardanol resin) were dissolved in 10 ml xylene in a 20 ml glass bottle equipped with a magnetic stirrer.

[0103] Both solutions were combined and heated at 135 C. under reflux, under continuous stirring until a homogenous solution was obtained. Which was then cooled at 100 C. under stirring and transferred into a 100 ml glass bottle. The solution was then further cooled at room temperature under manual stirring and crushed in high velocity homogenizer (Silent Crusher M from Heidolph) during 3 min, during which the crusher velocity was slowly increased from 5000 rpm to 12000 rpm in the case of BIG solution and from 5000 rpm to 7000 in the case of SB solution.

[0104] 0.46 g of isophorone diamine curing agent were dissolved in 5 ml xylene in a 20 ml glass bottle, and the solution was combined with the above crushed solution. A further 2 minutes crushing cycle was carried out.

[0105] A spraying method was designed in order to obtain a heterogeneous coated surface with biphilic characteristics comprising superhydrophobic spots (comprising PP grains) dispersed on top of a hydrophilic surface (stainless steel). For this purpose, the distance of the spray nozzle from the target surface as well as the aliquot of solution were varied as per Table 2. This way of proceeding allowed to control the distance between superhydrophobic spots as well as the percentage of stainless steel substrate surface covered with superhydrophobic spots. The obtained coatings were allowed to cure, in an oven controlled at 60 C., during 16 hours. The microstructural aspects of the coatings were evaluated by optical profilometrysee Table 2.

[0106] In the above table:

[0107] DOT_OPS_BIG_HD means spots of superhydrophobic surface material BIG (as per Table 1) dispersed with high density on substrate surface;

[0108] DOT_OPS_BIG_LD means spots of superhydrophobic surface material BIG (as per Table 1) dispersed with low density on substrate surface;

[0109] DOT_OPS_BIG_ULD means spots of superhydrophobic surface material BIG (as per Table 1) dispersed with ultra-low density on substrate surface; and

[0110] DOT_OPS_SB_LD means spots of superhydrophobic surface material SB (as per Table 1) dispersed with low density on substrate surface

[0111] The heterogeneous or biphilic surface obtained is characterized by [0112] Gsize which stands for average equivalent diameter of the grains on sample surfacesee FIG. 1 [0113] minDist which stands for average of the minimum distance between grainssee FIG. 1 [0114] % SHS which stands for percent surface occupied by the spots of superhydrophobic surface material

[0115] These values are obtained by 5 pictures of each surface; a Matlab script is performed to evaluate the average and the standard deviation. An example is shown in FIG. 1.

EXAMPLE 3

Evaluation of Biphilic Surfaces of the Invention in Pool Boiling Applications

[0116] The boiling chamber (FIG. 2) is made of aluminium and several heaters are applied on it, in order to maintain constant thermal conditions for the water contained within the chamber. An internal heater (80W) is initially used to heat up the pure water. Moreover, three external heating tapes and a K-thermocouple are placed in the pure water are placed on the walls of the chamber and are connected to a PID controller in order to balance any potential thermal leakages. These external heating tapes work in conjunction with a cooling system (air coils), in order to control the temperature of the chamber at the desired level during the experiment. A heat flux meter with 3 embedded T-thermocouples (Captec, France) is placed between the copper and the tested surface. A pressure gauge is connected to the chamber to measure its pressure. The boiling chamber is connected to bellows in order to modify the internal pressure. All the thermocouples, pressure gauge and heat flux meter are connected to a computer using a data acquisition system (Agilent A34970A data acquisition/switch unit, USA). The measuring accuracy of the T and K-thermocouples is 0.5 K, accuracy of the pressure gauge is 5 hPa.

[0117] The test procedure is schematically shown in FIG. 2. Firstly, the chamber was vacuumed down to 70 mbar before adding water in order to remove air and adsorbed gases inside the box. When the chamber was filled by degassed water, the temperature of the complete system (chamber and sample) was increased up to the saturation temperature of the pure water at atmospheric pressure (100 C.). Then the saturation conditions of the pure water in the chamber (T.sub.ch=100 C. and P.sub.ch=101.3 kPa, T.sub.ch and P.sub.ch were measured by a K-thermocouple and a piezo-electric pressure sensor) and were maintained with the PID system after point b .Thereafter, only the temperature of the sample was gradually increased by a specific sample heater (A ceramic cartridge Acim Jouanin 6.532175 of 175 Watts in a copper housing). This first increase of the sample temperature is called 1 ramp (b-c) and was systematically performed as a blank in order to remove all the peculiarities of the initial conditions in the setup cell and on the surface of the sample. After reaching a sample temperature of 130 C., the sample temperature was decreased (c-d) back to 100 C. (saturation conditions of pure water). Finally, the sample temperature was increased again (points d-e). The thermocouples and pressure gauges are connected to a PC through a data acquisition system. The measuring accuracy of the T and K-thermocouples is 0.5 K, accuracy of the pressure gauge is 5 hPa.

[0118] The following samples were tested: [0119] FILM_OPS_H rough: invention coating showing .sub.ECA=139 [0120] DOT-OPS: biphilic surface according to the invention comprising SH spots and showing different minimum average distance of the grains (minDist) and different average grain size (Gsize) as per below table.

TABLE-US-00005 Sample name minDist [m] Gsize [m] % SHS FILM_OPS_Hrough 0 100% DOT_OPS_BIG_HD 62.75 29.19 9% DOT-OPS_BIG_LD 130.50 31.01 2% DOT_OPS_BIG_ULD 255.52 37.85 1% DOT_OPS_SB_LD 279.92 65.37 2%

[0121] The average grain size and average minimum distance as well as an evaluation (as per Example 1) of the percentage of surface covered by superhydrophobic spots for relevant coatings tested are graphically represented in FIG. 3.

[0122] For the biphilic surfaces, the presence of two degrees of wettability favour the onset of the pool boiling (on surface area of second degree of wettability) but at the same time inhibit the formation of a vapour film (on surface area of first degree of wettability) raising the CHT value compared to a surface completely covered by second degree of wettability (as the FILM_OPS_Hrough). The influence of the average grains size (Gsize) and average minimum distance (minDist) on the pool boiling onset temperature (T.sub.ONB) for the sample in the table above is shown in FIG. 4. The T.sub.ONB is the sample temperature (measure as average of the value of the 3 embedded T-thermocouples) during the sample temperature increase (second ramp d-e FIG. 2) at which the continuous formation of bubbles from the sample surface (typically in just 1 or 2 points) and the rising of bubbles due to the buoyancy are clearly visible. It is possible to note in FIG. 4 The homogeneous values of T.sub.ONB (2 C. of max T.sub.ONB difference) during the second ramp for biphilic surface (DOT_OPS) and the invention coating (FILM_OPS) that indicate the proper operating of the grains on T.sub.ONB reduction.

[0123] FIG. 5 shows the heat flux curves as a function of the sample temperature, for the various tested coatings. As can be seen, sample DOT_OPS_BIG_LD shows the best performance.

EXAMPLE 4

[0124] A thin film of hydrophilic epoxy (SR8500/SD8605) dissolved in THF and xylene was deposited on a stainless steel substrate by spin-coating of 0.5 ml solution at 3000 rpm during 2 min. In a second step, spots of OPS_BIG composition (table 1) were applied onto the surface by the spraying technique described here above for the preparation of biphilic surfaces (spray distance 120 cm, aliquot 0.25 ml5 times). The resulting coating was allowed to cure in an oven at 60 C. for 16 hours. It is understood that superhydrophobic (SH) grains are partially immersed and surrounded by the epoxy layer serving as a glue preventing the grains from detaching from the coated surface. As a consequence, improvement of the coating durability is expected. The sample prepared by this method is called DOT_OPS_BIG_HD_EPOX_HPi.

[0125] In a further experiment, a first layer of hydrophilic epoxy (SR8500/SD8605) dissolved in THF and xylene was sprayed on a stainless steel substrate. Next, glass microspheres (diameter1000 micron) were deposited on top of this first layer covering the complete surface under evaluation. In a third step, a homogenous film of the SH polymeric composition (OPS_BIG) was applied by spraying (Spray distance 50 cm aliquot 0.5 ml5 times). The resulting coating was allowed to cure in an oven at 60 C. for 16 hours. Finally, the top layer of SH polymeric coating was removed from the top of the glass microspheres, hence forming a generally porous structure with biphilic surface characteristics expected to promote pool boiling heat transfer. It has been found that such surface treatment is particularly resistant to abrasion and durable. The sample prepared by this method is called MS_1000_EPOX_HPi_OPS_B_FILM.

[0126] For comparison purposes, a stainless steel surface coated by a first layer of Hpi epoxy and covered with genuine glass microspheres (no additional SH coating) was also prepared. The sample is called herein after MS_1000_EPOX_HPi.

[0127] FIG. 6 graphically represents the evaluation of T.sub.ONB during the second ramp, with a procedure similar to that of example 3, for the relevant surface treatments. It appears that the presence of glass microspheres increases the temperature required to activate the boiling (T.sub.ONB). Indeed in normal conditions an insulating material on the boiling surface, such as glass for the microspheres, generates an increase of the superheat temperature to initiate pool boiling (MS_1000_EPOX_HPI). In contrast, the coating MS_1000_EPOX_HPi_OPS_B_FILM is capable of reducing T.sub.ONB down to a value close to the T.sub.ONB found for DOT_OPS in example 3, evidencing that the layer of hydrophilic epoxy does not reduce the effect of the super hydrophobic coating called DOT_OPS_BIG_HD_EPOX_HPi.

EXAMPLE 5

[0128] In this example, the samples C_OPS, C_OPS_F, C_OPS_NP_F are tested in a pool boiling experiment using methanol liquid. The preparation of these samples is described in example 1. As the comparator, SS_smooth (a stainless steel surface AISI 316L with a Ra<0.1 m) was also tested. Methanol is a working fluid used in many phase change heat transfer devices (for example loop thermosiphon or heat pipe system). The liquid used for this experiment is 99.5% pure methanol. The saturation temperature at ambient pressure of methanol is around 64 C. and it has a surface tension (at 20 C.) of 22.7 mN/m (compared to 72.8 mN/m for water).

[0129] The apparatus is shown in FIG. 7. The boiling chamber is made of PTFE. A first heater H1 (300 W) is initially used to heat up the methanol at saturation temperature. A PID system (OMEGA CN77000) connected with a thermocouple (T1) immersed in the liquid, controls the electrical power to the internal heater in order to maintain the liquid temperature at saturation condition during the experiment (Tch=Tsat=64.7 C.). The chamber is open to the ambient on one-side, in order to maintain the methanol in the chamber at ambient pressure Pch=Patm and in contact with air. A second heater H2 (75 W) increases the temperature of the sample, recorded by a thermocouple (Tw) at the bottom of the sample. The pool boiling phenomena is visualized by a high speed camera (Phantom v5.2m). The saturation conditions of the methanol in this chamber (Tch=64 C. and Pch=Patm, Tch were measured by a K-thermocouple) were maintained with the PID system (that controls the electrical power to heater H1) after point b. A heat flux meter with 3 embedded T-thermocouples (Captec, France) is placed between the heater H2 and the sample. All the thermocouples, and heat flux meter are connected to a computer using a data acquisition system (Agilent A34970A data acquisition/switch unit, USA). The measuring accuracy of the T and K-thermocouples is 0.5 K.

[0130] The test procedure is schematically shown at the top-right of FIG. 7. After filling the chamber with pure methanol at ambient temperature (around 20 C.) the temperature of the system (chamber and sample) was increased up to saturation temperature (64 C.). The saturation conditions in methanol are maintained after point b (T.sub.ch=64 C. and P.sub.ch=Patm) by the PID system. Indeed heater H2 increased gradually only the sample temperature (Tw>Tch=Tsat). FIG. 8 graphically represents the T.sub.ONB evaluation, according to a procedure similar to that of Example 3. The OPS coating reduced T.sub.ONB by about 11% as compared to a simple stainless steel surface. Improved results can be obtained by fluorination of the coating after the deposition of nanoparticles (C_OPS_NP_F). In this case T.sub.ONB is about 25% less than simple stainless steel surface.

EXAMPLE 6

[0131] This example describes a thermal cycle experiment in order to evaluate the maintenance of wettabilities properties and effect on T.sub.ONB reduction after a considerable number of thermal cycles. The following samples described above were tested: [0132] DOT_OPS_BIG_HD_EPOX_Hpi [0133] FILM_OPS_Hrough

[0134] FIG. 9 shows the apparatus used for the thermal cycle experiment: the two samples were immersed in pure water in a glass chamber. The temperature of the glass chamber was varied by an external recirculation of oil fluid around the glass chamber. The oil was sourced alternatively (using a timed valve) from two different baths (Julabo). The temperature of each bath was set in order to generate the temperature variation shown at the top-left of FIG. 9. A thermal cycle started with the water at saturation condition (T1=100 C.) at ambient pressure. This condition was maintained for 15 min and thereafter the water temperature (T1) was decreased to 80 C. (no saturation condition) and maintained at this value for 25 min. Thereafter a new cycle was started with T1 at saturation conditions (T1=100 C.). Each cycle lasted 40 minutes, and the cycles were repeated respectively 156 and 506 times.

[0135] A water reservoir was connected with the bottom of the water chamber in order to compensate for loss by evaporation. The water temperature (T1) was recorded during the test using a K-thermocouple inside the water chamber and a data logger to record it (Omega TC-8).

[0136] FIG. 10 shows the results of the durability test. A measurement of the equilibrium static contact angle (using a KrDSA 100) was carried out on FILM_OPS_H rough after 0-59-156 cycles (plot on the top in FIG. 10). The equilibrium static contact angle remains essentially constant (within the tolerance margin of contact angle measurement); this demonstrates that the wettability properties after N-cycles (156-506) are maintained. In addition, the evaluation of T.sub.ONB for both samples (DOT_-OPSS_-BIG_-HD_-EPOX_-Hpi and FILM_OPS_Hrough), according to a procedure similar to the one explained in Example 3, are presented in FIG. 10 (plot in the middle for FILM_OPS_Hrough and at the bottom for DOT_-OPSS_-BIG_-HD_-EPOX_-Hpi). No significant degradation of the effect of wettability on T.sub.ONB reduction (T.sub.ONB is almost constant for all N-cycle) has been noticed.